you can successfully use upto 1TB partitions / physical drives within PT 6.4.1. please be aware that raid is not a good idea with IDE or SATA drives. only SCSI. the reason for this is because of the support that's been relied upon since PT started using SCSI drives, it's been kept in until PT 7 where it's opened the doors to SATA and SAS with the invention of G5 powermacs, etc.
for the moment, I'm using 4 x 160gb drives mounted in a magma 4 slot chassis with 3 protools mix cards and it does fine for me.
though I wouldn't mind sorting out larger drives for show work, etc.
ed